The present invention relates generally to the measurement of high temperatures, and more particularly, to an apparatus for accurately measuring high temperatures in an enclosed environment.
In various metallurgical procedures, high-temperature furnacing often in the range of about 1800.degree. to 2700.degree. C. is required for sintering refractory materials, for achieving the desired alloying, and for affecting the densification and other physical properties desired of the material being treated in the furnace. In order to provide these desired properties of the refractory material being furnaced, the temperature within the furnace must be accurate to within a few degrees of optimum. Thus, accurate and repeatable measurements of the temperature in the furnace must be provided for furnace control in order to achieve satisfactory production of the materials being metallurgically processed in the furnace. For example, inaccurate temperature measurements may lead to furnace temperatures greater than required so as to be catastrophic or deleterious to the materials being treated or alternatively be excessively low so as to fail to provide the material treated with the desired properties. This need for repeatable furnace temperature measurements in the aforementioned temperature range, is believed to be of critical importance in high-temperature metallurgical processes.
Previous experience has shown that optical apparatus for measuring temperatures provide a relatively simple and stable mechanism capable of being relatively easily calibrated so as to provide high sensitivity in the temperature range of about 1800.degree. to 2700.degree. C. required for repeatable temperature measurements in metallurgical furnaces. A typical measurement using optical procedures utilizes a broadband multicolor radiometer which is capable of providing accurate temperature measurements in the aforementioned range since the output from such a radiometer varies as the fourth power of the difference in absolute temperature between the temperature source and the detector. With such an output, the maximum temperature measurement sensitivity is obtained at the higher temperatures so as to make the optical procedure using a broadband radiometer most advantageous for high-temperature measurements. Alternatively, a monochromatic single color radiometer may be utilized for obtaining high-temperature measurements but such a system is of significantly greater cost and does not increase the reliability of the system to a level justifying the increased expenses of using such a system.
While optical temperature measuring systems are satisfactory for obtaining the measurement, some problems occur which detract from the overall efficiency and desirability of the system. In a typical operation of an optical measuring system, the thermal radiation of a blackbody within the furnace is optically measured with this thermal radiation being transmitted along a sight path from the blackbody to a radiometer. Normally, sighting tubes are used for placing the blackbody in optical alignment with the radiometer so that accurate temperature measurements may be achieved. However, when a furnace interior such as a graphite induction furnace is subjected to temperatures greater than about 2,000.degree. C., a considerable quantity of radiation attenuators such as gases, soot, carbon particles, and various impurities emanating from attendant graphite structures cloud the sight path so as to detract from the accuracy of the measurement. Also, since these attenuators are not uniformly emitted, the level of the thermal radiation reaching the sensing device or radiometer varies so as to considerably detract from the efficiency of the temperature sensing system. Efforts to remove these radiation attenuating materials from the sight tube by continually purging the sight tube with a gas have not shown to be practical in that the gas has some cooling effect upon the blackbody and the sighting tube so as to result in inaccurate measurements of the temperature in the furnace.